I. Field of the Invention
This invention relates to an automatic reactor for catalytic microactivity studies, based on a hot box system, which is especially conceived for studying the behaviour of a catalyst in the presence of some reactants and how this alters the activity and selectivity of a defined chemical reaction.
The automatic reactor proposed by the invention is in the field of two different sectors, industry and research. At the industrial scale this type of equipment will be able to be used primarily in the search for suitable catalysts for a process and obtaining of the optimum operating parameters. At the research level, it will primarily be used for evaluating activity and selectivity in the development of new catalysts.
The object of the invention is therefore to provide a reactor which achieves certain levels of repeatability, reliability of results and automation much higher than those obtained by commercial systems currently in existence.
II. Description of the Related Art
In the field of heterogeneous catalysis it is very complicated to predict the catalytic behaviour of a material solely on the basis of its composition and/or chemical structure. For that reason, the only way to determine whether such a material possesses good catalytic properties for a chemical reaction is to conduct an activity test under the same operating conditions as the process of interest, which usually lie in the following intervals:                Pressure: 10-90 bar        Temperature: ambient—750° C.        Charge: Liquid and/or gases: Stream of reactants: 0.01-100 VPH (volume of charge per unit of catalyst and per hour) for liquids and 10-100,000 for gases.        Solid catalyst (balls, pellets, extruded particles, etc.).        
When the aim is to measure the catalytic activity of a solid under conditions close to or identical with those used in the industrial process, the pilot plants that are currently available display a series of difficulties, especially in situations in which one is working at pressure above atmospheric:                Large quantities of catalyst are required, which implies a serious obstacle when its preparation is complicated or very costly.        The activation and stabilisation of the system under the desired operating conditions take long periods of time (hours, or even days), due to the thermal inertia of the equipment and dead spaces.        The majority of the usual facilities do not permit automatic on-line analysis with the reactor which would entail continuous analysis and short response times.        The complete automation of the system of a standard facility and its control by computer is a very complex and a highly expensive operation.        The achievements in accuracy and reliability in this type of equipment are limited, due both to reasons that are intrinsic to the system (lack of stability of the catalyst in very large time periods) and also extrinsic (difficulty when it comes to having a system free of fluctuations when modifying the reaction parameters).        
This need to conduct activity tests under conditions identical to those used in industrial reactors has led to the development of microactivity reactors, with associated systems of sampling and analysis of the reaction products which make it possible to obtain a rapid response from the system in each catalytic evaluation. This objective is achieved by means of a reduction in the reaction times, associated with the small quantities of catalyst required and the small dimensions of the reaction equipment.
In the majority of equipments with which one works at atmospheric pressure, the liquid/gas separation is done by passing the reaction products through a refrigerated tank, like that described in regulation ASTM D3907-80 for the MAT of FCC catalysts, and sometimes exerting a slightly negative pressure of −60 mbar, subsequently quantifying the reaction liquids by weighing and quantifying the gases in a totaliser by displacement of water. In patent ES9000012, J. Prieto, A. Corma and F. Melo describe an automated MAT reactor for carrying out consecutive sequences of reactions without the presence of an operator.
But in microactivity equipment working at pressures above atmospheric, it is necessary to determine the compositions of the output effluents from the reactor at the reaction temperature and pressure. The separation of phases is currently carried out inside a liquid-gas condenser/separator capable of performing this operation at high pressure, thereby increasing the efficiency of the system. With the aim of avoiding the accumulation of liquids inside the separator, a reading of the level of liquids permits control over it, with continual evacuation of as much liquid as is condensing in the system in real time. This operation is currently carried out in commercial equipment by measurement of the differential pressure in the condenser, permitting indirect measurement of the level. But, owing to the dead volume associated with the measurement in the separator, the limitations associated with this technique are very considerable, since in a catalytic microactivity reaction, where the streams of liquid are of the order of 0.05 ml/min, the first drop of sample product of the reaction would be evacuated from the system hours after the start of the reaction. So, with this system it is impossible in real time to have samples of the reaction products at very short reaction times (of great interest for studies of reaction kinetics) and without contamination as a consequence of dilution inside this separator.
At the outlet from the condenser, and once the phases have been separated, a valve, associated with a pressure sensor, acts on the gas stream evacuating from the reactor, keeping the pressure of the system constant. But this operation, carried out with the pressure control devices commercially available right now, is done at the cost of generating some characteristically pulsating gas streams in the reaction system, which has an appreciable effect on the reproducibility and reliability of the results obtained. Certain electronic elements for pressure control upstream available on the market manage to get around these effects, but their use is not possible when there exist condensable vapours in the system.
Once the separation has been carried out, the reaction gases are analysed by gas chromatography, either in a subsequent stage or by means of on-line analysis. One of the limitations of this type of analytical system is the long analysis times that are required, which makes it impossible to have analysis at short reaction times. As a possible solution to this problem, H. Ajot (U.S. Pat. No. 5,266,270 A) developed a catalytic microactivity unit with a system of sampling valves, provided in the furnace, in series and parallel, at the outlet from the reactor, which would permit the gathering and storing of outlet effluents in different compartments of known volume at high temperatures (from 50 to 300° C.), avoiding the condensation of heavy products. This system allows reaction samples to be taken at reaction times as short as is wished in order later on to analyse them successively by gas chromatography.
P. M. Michalakos et al (Catalysis Today 1988, 46, 13-26) applied a similar system of sample taking to a microactivity reactor for fluidised bed catalytic cracking reactions based on standard ASTM D3907, to which he coupled a system of ten cold traps (MCT system, Multiple Cold-Trap, Chevron patent) in which the products were condensed and stored at the desired reaction times for their later evaporation and analysis by gas chromatography. In both cases, the reaction sample is taken without separation of liquids and gases, and is advantageous when it comes to testing catalysts which are rapidly deactivated at small reaction times. But in neither of these two sampling systems that have been developed is it possible to carry out a strict material balance, since there is no on-line analysis available of the effluents in the reaction conditions under pressure. Moreover, the monitoring of the catalytic reaction continues to be limited and conditioned by the long analysis times that are required in this type of system. So, it is impossible to act on the reaction parameters as a function of the analyses obtained in real time.
On the other hand, E. C. Milberger, in U.S. Pat. No. 4,099,923, describes an automatic unit for the preselection of catalysts, obtaining a preliminary indication of its potential activity. H. Kögler in DE 2 425 227 describes an automatic microreactor for studying catalytic reactions under pressure. But none of these documents suggests carrying out a complete material balance of the chemical reaction under pressure, determining the outlet stream from the reactor at its pressure and temperature, a measurement which cannot be made in any direct way.
There are few highly automated microactivity equipments described in the literature. On the contrary, most of them have a very low level of automation, requiring close attention and dedication by the operator. Works published in 1998 can be highlighted, in which the incorporation of a computer that would control the process was predicted for the future, and mention can also be made to U.S. Pat. No. 5,266,270 which incorporated programmable controllers into a MAT type reactor, the automation of which permitted good reproducibility and reliability in the results obtained.
In the catalytic microactivity reactor described in U.S. Pat. No. 6,497,844, the control of the streams of reactants is done by means of valves, and the control of pressure and temperature in the system is done by means of PID controllers, with the reference point being able to be transmitted by an RS-485 type connection. The system is provided with safety measures in the event of a failure in the system, so that if a regulating element (thermocouple, valve, pressure sensor, etc.) fails, the controllers cancel the specified reference point and the system stops in a “safety” status. The elements of the system are connected to a computer in such a way that the software used guarantees operation in sequential mode, along with recording of the different parameters of the process. But this system, controlled solely by means of computer, does not provide for control of the system in the event of a failure in the computing system, which would leave the system out of control.
So, as has been confirmed, current systems display drawbacks such as the high cost of this type of equipment on the market, even when their performances are low and their level of automation is relatively poor.